This invention relates to a method for preparing tertiary phosphines from the corresponding phosphine oxide. More specifically, this invention relates to the reduction of tertiary phosphine oxide using halosilanes under conditions considered undesirable by the prior art.
The prior art teaches the reduction of tertiary phosphine oxides by halosilanes in the presence of tertiary amines. Specifically, U.S. Pat. No. 3,261,871 discloses that a tertiary phosphine oxide can be reduced by reacting from 0.7 to 1.5 moles of a halosilane containing the residue ##STR1## and from 0.7 to 1.5 moles of a tertiary amine for each mole of the tertiary phosphine oxide. This patent teaches that the amine is employed to react with the hydrogen halide formed as a by-product during the reaction. Primary and secondary amines are known to displace halogen atoms bonded directly to silicon to form aminosilanes. In accordance with this teaching of the aforementioned U.S. Pat. No. 3,261,871, primary and secondary amines would be unsatisfactory acid acceptors since the one or two hydrogen atoms bonded to the nitrogen of the amine are sufficiently reactive to displace halogen as a hydrogen halide from the silane reagent and form an aminosilane which may not be capable of reducing the phosphine oxide.
An objective of this invention is to provide a method for increasing the yield and purity of tertiary phosphines formed by reacting the corresponding phosphine oxide with a halosilane. It has now been found that this objective can be achieved if the reaction is carried out in the presence of at least a stoichiometric amount of a primary or secondary amine, which is contrary to the teaching of the prior art.